A double peaked pulse profile observed in GX 1 + 4

Abstract

The hard X-ray pulsar GX 1+4 was observed several times in the last few years with a pair of balloon-borne Xenon filled Multi-cell Proportional Counters (XMPC). In a balloon flight made on 22 March 1995, the source was detected in a bright state, the average observed source count rate being 8.0 ± 0.2 /s per detector. X-ray pulsations with a period of 121.9 ± 0.1s were detected in the source and a broad double puslse features the pulse profile. This double pulse structure is observed for the first time in this source in high energy band and indicates activation of the opposite pole of the neutron star. Compared to our previous observation of the same source with the same telescope a period change rate of 0.72 ± 0.40 is obtained which is the lowest rate of change rate of period for this source since its discovery. A reversal in the spin change, from spin down to spin up in the mean time is consistent with our period determinations. BATSE observations in this period have also indicated the same trend. Pulse fraction in the hard X-ray range is low (30%), consistent with its anti correlation with luminosity and the observed spectrum is very hard (power law energy index 0.67 ± 0.12). Power law spectrum, thermal Bremsstrahlung and two Compton scattered Bremsstrahlung models are fitted well with the spectrum. No indication of change in the spectrum with the pulse phase is observed. The observed source luminosity in the 20 - 100 keV range is 2.5 x 10 to the power 37 erg /s. Pulse phased spectra did not show any indication of change in the hardness with phase.